In the photolithography step of integrated circuit manufacturing, a template containing a designed set of clear and dark shapes, referred to as a mask or reticle, is repeatedly printed on the surface of a silicon wafer via optical imaging. The minimum resolution R, defined herein as the smallest center-to-center separation for which two individual objects can be resolved on the wafer (also referred to as the minimum resolvable pitch), is given by the expression: EQU R=2.times.k1.times.(.lambda./NA), (1)
wherein NA is the numerical aperture of the optical projection system (hereinafter referred to as the stepper) and .lambda. is the wavelength of the light used by the stepper. For partially coherent imaging, the theoretical limit for the resolution, as defined above, occurs at a k1 factor of: EQU k1=1/2.times.(1+.sigma.)!, (2)
wherein .sigma. is the partial coherence factor of the stepper.
In standard industry practice as outlined in the schematic mask cross-section of FIG. 1, masks containing the desired opaque and clear patterns are fabricated starting from an initial mask blank (FIG. 1A) consisting of a substrate which is transparent to the imaging light 10 and which is coated on one side with an opaque film 20. Typically, the transparent substrate is composed of fused silica (also known as quartz), hereinafter referred to as the quartz substrate, while the transparent substrate material will be referred to as quartz. In addition, the opaque film is typically composed of a chromium-based material to be referred to as the chrome film, while the material of the film itself will be referred to as chrome. The designed shapes are replicated on this mask blank by first selectively patterning (or writing) the designed shapes in a protective material sensitive to electrons or optical exposure (FIG. 1B), and which will be referred to as resist 30. The openings created in the resist via selective patterning 35 are then transferred to the underlying chrome film during a subsequent etch step such that following removal of the resist material (FIG. 1C), the designed clear shapes 40 and opaque shapes 21 are replicated in the final patterned mask. Masks fabricated in this fashion will be referred to hereinafter as standard or chrome-on-glass (COG) masks.
A different class of masks, such as a phase-shifting mask (PSM), has demonstrated the capability of extending resolution beyond conventional imaging limits by taking advantage of both the phase and the intensity of the imaging light. If two clear shapes which transmit light of opposite phases (180.degree. phase difference) are moved in close proximity to each other, the phase difference will produce a destructive interference null between the two shapes. Such a mask has been given several different designations in the literature such as: Levenson, Levenson-Shibuya, phase edge, alternating aperture, or alternating mask. Herein, it will be referred to as an alternating mask or as an alternating PSM. With this alternating PSM approach, the theoretical resolution limit is reduced to: EQU k1=1/4.times.(1+.sigma.)! (3)
or one-half the resolution of conventional imaging.
The additional mask fabrication steps beyond the standard mask process of FIG. 1 are shown for an etched-quartz or subtractive alternating PSM process in FIG. 2. A phase difference between two clear shapes for an alternating PSM is achieved in standard industry practice by selectively etching into the quartz substrate 10, such that an optical path difference equivalent to the desired phase offset is obtained between two adjacent openings. Following a standard mask patterning as shown in FIG. 1, a second write step is used to selectively open a protective resist coating 50 for the phase-shifted opening 41 leaving the non-phase shifted opening 42 covered, as shown in FIG. 2A. In practice, it is desirable to locate the edges of the resist pattern 55 some distance away from the phase-shifted opening 41 on top of the opaque chrome shapes 21 where appropriate, in order to use the chrome itself as an etch barrier and account for overlay (or pattern placement) errors between the first and second-level write steps in the fabrication process. The quartz is then etched (typically, with an anisotropic reactive-ion etch (RIE) process) to a depth of approximately: EQU etch depth=phase.times./2.times..lambda..times.(n-1)!, (4)
wherein n is the refractive index of the quartz substrate, and the phase of the opening 41 is given in radians. Following removal of the resist 50, the resultant alternating PSM has, as shown in FIG. 2B, the etched-quartz trench 15 providing the desired phase difference between adjacent openings 41 and 42.
More complex fabrication approaches have been proposed for accurately controlling the phase (as determined by equation 4) through the addition of multi-layer films to the transparent substrate. By way of example, T. Chieu et al., in the article "Fabrication of Phase Shifting Masks Employing Multi-Layer Films," Proc. SPIE, Vol. 2197, pp. 181-193, (1994) describe a mask blank wherein two additional layers are added between the transparent quartz substrate and the opaque chrome: an etch stop layer composed of either Al.sub.2 O.sub.3 or HfO.sub.2, and a transparent layer of silicon dioxide at a controlled thickness given by equation (4). The desired phase is then achieved by etching into the SiO.sub.2 until the etch stop layer is reached.
The invention to be described herein is applicable to both dark-field and bright-field patterns which are defined in FIG. 3. In prior-art dark-field designs (a majority of the area is designed to be opaque on the mask), the closed shapes within the design (i.e., polygons of any number of vertices and edges, although rectangles are used herein for simplicity) typically represent clear openings in the chrome of the mask, shown by 41 and 42 in the cross-section of FIG. 2B and in the design data of FIG. 3A. In addition, for an alternating PSM, the openings which are to receive the quartz etch required to establish phase 41 are typically indicated by including a surrounding shape that corresponds to the second writing step in the fabrication process, shown by edge 55 in the cross-section of FIG. 2A and by shape 55 in FIG. 3A. For bright-field designs, an example of which is shown in FIG. 3B, (a majority of the area is designed to be transparent on the mask), the closed shapes within the design typically represent remaining regions of opaque chrome as represented by the chrome features 21 in the cross-section of FIG. 2B and the rectangles 21 in the design data of FIG. 3B; the clear openings in the chrome such as 41 and 42 in the cross-section of FIG. 2B are then given by the spaces between the designed shapes 41 and 42 in the design data of FIG. 3B. With a bright-field alternating PSM, the shapes describing the second-level write step for defining the areas for the phase etch 55 in both FIGS. 2B and 3B, will both overlap the designed chrome shapes as well as contain bare edges within the clear areas of the design 56, as shown in FIG. 3B. These bare phase edges may print as a narrow dark line which may require subsequent removal via techniques well known to those skilled in the art, such as the use of a second (trim) mask. The basic principles of this patent are unaffected by the specific technique chosen to remove the residual phase edge in bright-field alternating PSM lithography.
The fabrication process illustrated in FIG. 2 has proven to yield non-ideal results when used to photolithographically image such mask patterns into the photo-sensitive resist (also known as photoresist) on the wafer substrate. The ideal results obtained from printing the mask of FIG. 2B are shown schematically in FIG. 4A. The photoresist 63 on wafer substrate 60 contains two equally-sized openings 61 and 62 corresponding to the clear openings on masks 41 and 42. (Note that for illustrative purposes, a positive photoresist in which the photo-sensitive material is removed in areas that are exposed to light is assumed, although the invention is applicable to negative photoresist as well). In practice, however, it has been conclusively demonstrated that the edges of the etched-quartz trenches 15 of an alternating PSM scatter the incident illumination. This, in turn, leads to a reduced intensity being transmitted through the phase-shifted mask opening 41 (in FIG. 2b) relative to that transmitted through the non-phase-shifted mask opening 42 (in FIG. 2b) during the photolithographic exposure. The asymmetric printing of the desired pattern that results from this transmission error is schematically illustrated in FIG. 4B, wherein the resultant photoresist opening 71 corresponding to the phase-shifted mask opening 41 is undersized in comparison to the adjacent photoresist opening 72 corresponding to the non-phase-shifted mask opening 42. In conjunction with the dimensional error, the center of the photoresist pattern between the two openings 73 has shifted to the right resulting in a pattern placement error as well. The impact of sidewall scattering on transmission error has been substantiated both through simulations of electromagnetic scattering via rigorous solution of Maxwell's equations and through experimental verification as described in an article by R. Kostelak et al., published in the Journal of Vacuum Science and Technology B, Vol. 10(6), pp. 3055-3061, (1992).
Several methods have been described to correct this transmission error. A summary and detailed analysis of these approaches can be found in an article entitled: "Pattern-Dependent Correction of Mask Topography Effects for Alternating Phase-Shifting Masks", by R. A. Ferguson, et al., published in the Proceedings of SPIE, Vol. 2440, pp. 349-360, (1995). Two approaches are discussed herein as relevant prior art to this invention. The first technique, as described schematically in FIG. 5A uses a quartz etch-back process to alleviate the transmission error. Following the standard processing sequence illustrated in FIGS. 1 and 2, the reticle is subjected to an isotropic etch (e.g., immersion in dilute Hf solution) in which the sidewalls of the etched quartz trench 15 are recessed beneath the chrome film 22. The non-phase-shifted opening is also etched simultaneously in this manner such that the relative phase difference as defined by equation (4) remains constant. While this process has been effective at reducing the transmission error, rigorous simulations indicate that complete removal of the transmission error cannot be attained as described in the aforementioned article by R. Ferguson et al. In addition, the overhang of the chrome as indicated by 22 in FIG. 5A can become significantly large (e.g., in excess of 1000 .ANG.), rendering the physical stability of the overhanging chrome questionable under standard manufacturing conditions. This is especially true when the mask undergoes vigorous cleaning procedures that are standard and an essential component of producing and maintaining zero-defect, manufacturing-quality masks.
In a second approach described by A. Wong and A. Neureuther in the article entitled "Mask Topography Effects in Projection Printing of Phase-Shifting Masks", published in IEEE Transactions on Electron Devices, Vol. 40, No. 6, pp. 895-902, (1994), the initial design data (FIG. 3) is manipulated prior to mask fabrication such that when it is coupled to the standard fabrication process of FIGS. 1 and 2, the mask shown schematically in FIG. 5B is obtained. In the aforementioned design modifications, the phase-shifted opening 41 has been enlarged by a preset bias relative to the non-phase-shifted opening 42 in order to increase the transmitted light through the phase-shifted opening 41 and, thus, compensate for the transmission loss from side-wall scattering from the edges of the etched-quartz trench 15. In theory, this approach can fully compensate for the scattering phenomena when the design bias is continuously adjustable. In practice, however, the range of allowable biases is limited to discrete values on a coarse grid (referred to hereinafter as the design grid). While a reduction in the step size of this finite design grid will provide more ideal characteristics, the lower bound that is placed on the step size of the design grid by the writing tool used to pattern the mask limits this technique to unacceptably large residual transmission errors as described in the aforementioned article by R. Ferguson et al.